1. Technical Field
The present invention relates to transformers which include a plurality of coils that are electrically insulated from each other and stacked in a common winding axial direction thereof.
2. Description of the Related Art
There are known transformers which are used in, for example, DC-DC converters. Those transformers include, as shown in FIG. 30, a high voltage-side coil 91 and a pair of low voltage-side coils 92 that are electrically insulated from each other and stacked in a common winding axial direction thereof (i.e., the direction of the winding axes of the coils 91 and 92 which coincide with each other). More specifically, the coils 91 and 92 are stacked so that the high voltage-side coil 91 is interposed between the low voltage-side coils 92 in the winding axial direction. Further, the coils 91 and 92 are together sandwiched by a pair of core pieces 93 in the winding axial direction. With the core pieces 93, magnetic paths can be formed on both the radially inside and radially outside of the coils 91 and 92.
Moreover, electrical insulation between the high voltage-side coil 91, the low voltage-side coils 92 and the core pieces 93 is secured by interposing therebetween bobbins 94 that are made of an electrically-insulative material. However, with the bobbins 94, both the size and parts count of the transformer 9 are increased and the assembly process of the transformer 9 is complicated.
To solve the above problem, Japanese Patent Application Publication No. 2004-303857 discloses a technique, according to which the high voltage-side coil 91 is comprised of a substrate that has coil patterns formed on both the major surfaces thereof and insulating layers 911 that cover the coil patterns. Consequently, the high voltage-side coil 91 is electrically insulated from the low voltage-side coils 92 without interposing the bobbins 94 between the high voltage-side coil 91 and the low voltage-side coils 92.
However, with the above technique, it is still necessary to interpose the bobbins 94 between the low voltage-side coils 92 and the core pieces 93 for securing the electrical insulation therebetween. Consequently, it is difficult to minimize both the size and parts count of the transformer 9 and simplify the assembly process of the transformer 9.